Technical Field
The present invention relates to a flux-cored wire for carbon dioxide gas shielded arc welding which provides excellent welding workability in all-position welding when steel used for a steel structure or the like is welded and weld metal having excellent characteristics such as low-temperature cracking resistance, low-temperature toughness, and fracture toughness (hereinafter, referred to as CTOD).
Related Art
As a flux-cored wire used for gas shielded arc welding using steel as a material to be welded, for example, a rutile type flux-cored wire or a basic flux-cored wire is known. Welding using the basic flux-cored wire can reduce an oxygen amount of weld metal, and therefore the weld metal has excellent low-temperature toughness and CTOD characteristic. However, welding using the basic flux-cored wire has poorer welding workability in all-position welding than welding using the rutile type flux-cored wire, and therefore is not often used generally.
On the other hand, carbon dioxide gas shielded arc welding using the rutile type flux-cored wire provides extremely excellent welding efficiency and welding workability in all-position welding, and therefore is applied in a wide range of fields such as shipbuilding, bridges, oceanic structures, and steel frames.
However, the rutile type flux-cored wire is obtained by filling a flux mainly including a metal oxide such as TiO2 into a steel outer skin, and therefore weld metal has a large amount of oxygen and does not easily obtain low-temperature toughness. Particularly when a CO2 gas is used as a shielding gas, it is more difficult to secure toughness than a case in which a mixed gas of Ar and CO2 is used. In addition, an amount of diffusion hydrogen is larger than that in a solid wire due to moisture included in a raw material of the flux or moisture absorption while the wire is stored. Therefore, there is a risk of low-temperature cracking of weld metal. It is necessary to perform preheating at about 100° C. when a thick steel plate is welded. This reduces a welding efficiency.
Various developments have been performed for a flux-cored wire for carbon dioxide gas welding of low-temperature steel. For example, JP 2009-61474 A discloses a technology of adding an alloy component such as Ti which changes into a slag component during welding in order to obtain weld metal having excellent low-temperature toughness by reducing an oxygen amount of the weld metal while an amount of slag which prevents dripping of melted metal (hereinafter, referred to as metal dripping) in vertical upward welding is maintained by adding the alloy component which changes into the slag component during welding. However, in the technology described in JP 2009-61474 A, an arc state is unstable, a generation amount of spatters is large, sufficient low-temperature toughness of weld metal is not obtained, and low-temperature cracking resistance is not taken into account even though high-temperature cracking resistance is secured.
JP 2002-361486 A also discloses a technology for obtaining weld metal having excellent low-temperature toughness. In the technology disclosed in JP 2002-361486 A, an oxygen amount of weld metal is reduced and welding workability in all-position welding is excellent by maintaining addition amounts of TiO2, B, Mg, and Al properly. However, Al added as a strong deoxidizer remains excessively in weld metal as an Al oxide, and low-temperature toughness or an excellent CTOD value of the weld metal is not obtained. In the technology disclosed in JP 2002-361486 A, low-temperature cracking resistance is not particularly taken into account.
JP 2014-113615 A discloses a technology for obtaining welding workability in all-position welding and low-temperature toughness by including a slag component containing a metal oxide mainly containing TiO2 and a metal fluoride, an optimum alloy component, and a deoxidizer. However, also in the technology disclosed in JP 2014-113615 A, an oxygen amount of weld metal is large due to carbon dioxide gas shielded welding, and an excellent CTOD value is not obtained particularly at a low temperature, disadvantageously.